A glass cube is floating horizontally in outer space. Two rays of starlight coming from the left and right are passing through the glass horizontally. Below is the difference between the two rays of light as seen from viewpoint of the glass (difference in formula v = f λ).
Difference inside the glass c/n = c/n f ≠ f λ ≠ λ
Difference just before entering the glass. c ≠ c f ≠ f λ = λ
Difference just after leaving the glass c = c f ≠ f λ ≠ λ
